1. Field of the Invention
The present invention concerns a method and a magnetic resonance system to generate a series of temporally successive angiographic magnetic resonance images of an examination region, in particular a contrast agent-free generation of temporally successive angiographic magnetic resonance images.
2. Description of the Prior Art
Various methods for contrast agent-free dynamic magnetic resonance angiography, what is known as non-contrast enhanced (NCE) magnetic resonance angiography, are known in the prior art. In these methods, blood flowing in is marked with the aid of what is known as a spin labeling method and imaged during the inflow into an examination region, known as an imaging volume of interest (VOI).
To mark and depict the inflowing blood, the same examination region is typically acquired twice with different labeling. In the prior art the methods FAIR (Flow-Sensitive Alternating Inversion Recovery) and STAR (Signal Targeting with Alternating Radio Frequency Sequences) are known in this regard. In the FAIR method a first measurement is implemented in a first measurement after a selective inversion (marking) of the VOI and a second measurement is implemented after a non-selective inversion of VOI and the surrounding region. In the STAR method a region outside of the VOI from which blood flows into the VOI is inverted before a first measurement, and a second measurement occurs without marking. In both methods inflowing magnetization portions in the individual measurements respectively show a different signal and therefore can be represented by a subsequent calculation of the difference of the first measurement and the second measurement, while static regions respectively respond identically, and this static measurement signals are largely eliminated by the subtraction. In this method an image is typically respectively acquired after a specific inflow time. These methods can be used for NCE magnetic resonance angiography by repetition with different inflow times. In order to improve the efficiency of the method, an entire desired observation period of time after the marking can be scanned with a series of images (known as frames) in order to thus acquire the workflow of the inflow process with time resolution. Gradient echo methods can be used for this, for example FLASH imaging (Fast Low Angle Shot) or FISP or, respectively, TrueFISP imaging (Fast Imaging with Steady State Precession, also known as b-SSFP (Balanced Steady State Free Precession)).
In such a continuous scanning of the inflow process, the chronological development of the magnetization (and therefore of the signal) is significantly affected by the permanently active measurement. The marking—i.e. the difference between the signal values of the inflowing blood in the two measurements—disappears with time, such that the blood signal is always weaker in the difference data. Vessels that only fill late with the marked blood after the marking are thereby not (or are only insufficiently) acquired and shown. This loss of the marking takes place particularly quickly in FLASH imaging, but also represents a severe limitation in FISP imaging (Fast Imaging with Steady State Recession).
In the prior art the “marking edge” is therefore placed near to the vessels of interest in order to acquire the inflowing blood as well as possible in these regions. However, the volumes within which the inflow process can be acquired is hereby strongly limited. This is critical in the examination of pathologies with unclear flow relationships (for example the examination of arterio-venous malformations) in which it is precisely the question of vessels leading to and away that should be clarified.